Piezo-electric devices



Aug. 21, 1956 R. H. STURM 2,759,241

PIEZO-ELECTRIC DEVICES Filed OG'C. 14, 1952 United States Patent OPIEZO-ELECTRIC DEVICES Robert Heinrich Sturm, Enfield, EnglandApplication Gctober 14, 1952, Serial No. 314,615

Claims priority, application Great Britain October 17, 1951 2 Claims.(Cl. 29-25.35)

This invention relates to piezo-electric devices of the type employingtwo or more layers of material which are secured together.

A well known piezo-electric assembly, widely used in transducers such asmicrophones, gramophone pickups, sound reproducers and like, is abirnorph construction in which two plates of a piezo-electric substance,having electrodes on their surfaces, are cemented together. Morecomplicated constructions using three or more piezo-electric elementssecured together are also known; for the purpose of this presentspecification the term bimorph is to be taken as including not only thetwo element assembly, but also the more complicated ones.

The crystal plates which are used in bimorphs are often very thin:commonly they are of the order of 25 to 30 mils thick. Since theseplates have to be ground or machined to this thickness, care must beexercised to avoid undue losses due to breakage of the plates duringmachinmg.

The present invention has for its object to provide an improved methodof machining or grinding the plates of bimorphs, and in accordance withthe invention one side of a crystal element is ground and the member isthen stuck by this face to a thin sheet; whilst held by this sheet upona support surface the opposite side of the element is ground ormachined. The supporting sheet permits the crystal element to be held byvacuum pressure upon the support.

In the specification of my co-pending application Serial No. 314,563tiled October 13, 1952 we have described a method of moisture-proofingbimorphs, which involves the steps of first preparing one surface ofeach crystal element, providing an electrode means on that surface andthen applying a moisture-proof coating to the surface; thereafter twocrystal elements prepared in this way are cemented together. Thecementing can be effected by means of a sheet or layer of materialchemically aliine to the material of the coating, used as aninterlaminate between the two plates to be secured together. The presentinvention can be used with advantage with such a method.

Other features and advantages of the invention will appear from thefollowing description of one embodiment thereof as applied to themanufacture of bimorphs suitable for use in a transducer such as amicrophone, gramophone pickup, sound reproducer or the like.

In this process sheets of Rochelle salt are cut from a crystal slab inknown manner, for example by means of a band saw. These plates are asthin as may be produced in this way Without undue loss due to breakages;a thickness of about 0.1 of an inch is obtainable in practice.

The sheets are then cut in rectangular plates having a predeterminedorientation with respect to the electrical axes of the crystal, therectangles being slightly larger than the desired linal size of theplate.

The edges of the plates are lightly ground to finish them to the desiredsize, and one of the major surfaces of each plate is then machined at(assuming, as is almost invariably the case, the bimorph is flat). Thismachining Patented Aug. 21, 1956 ICC can be carried out safely with acrystal plate of the thickness mentioned; the plate can be temporarilyattached to a work support by a viscous material, and the surface truedby grinding, milling or turning.

The plates are next cleaned; it is important that the machined surfacesshould be free not only of ordinary foreign matter, but also any minutecrystal of the Rochelle salt itself, since these react more quickly toatmospheric moisture than the monocrystalline plate, due to their smallsize and proportionally large surface. These small particles thereforeincrease the possibility of deterioration of the finished bimorph due tothe ingress of moisture, if these are permitted to remain on thecrystal.

The trued major surface of the crystal is next provided with anelectrode; various methods of providing electrodes can be used includingvacuum evaporation, electroplating or the application of a metal foil,but we prefer to make use of a metallic coating material consisting ofextremely tine metallic silver particles, of 750 mesh size, suspended ina solution of preferably synthetic resin, such as for example an acrylicresin in a volatile solvent such as toluene, Xylene or the like. This isbrushed onto the surface, a foil strip terminal being attached, and isallowed to dry. There results an extremely closely adherent conductivelayer of material consisting of about metallic silver. The Very closeadherence is of importance inasmuch as it ensures a high capacitance forthe finished bimorph.

When the electrode is dry the plate is coated with a high dielectricmaterial having low moisture absorption; this material is of the typedisclosed in the copending application referred to, namely, fully orpartly polymerised polyester resins such as diallyl phthalate,polystyrene resins, polyvinyl resins, polyvinyl copolymer resins,polyaryl ethylene oxide resins, polyacrylic resins, chlorinated diphenylresins alone, or a mixture of a coplymerisation product of such resinswith or Without the addition of modifying agents such as plasticizerslike dibutyl or di-octyl phthalate, tri-cresyl phosphate or dibutylsebacate, tackifying resins such as ester gums, polyisobutylene resins,abietic acid esters, llers and pigments such as talc, silica or titaniumdioxide, and catalysts such as benzoyl peroxide, tertiary butylhydroperoxide, pentaethylene diamine.

The coating may be limited to the machined surfaces of the plates, butit is easier to give a complete coating. The coating is of the order ofhalf a mil in thickness. The plates are coated and dried at atemperature of about 20 to 22 C., at a relative humidity of 40 to 50%.

The plates are then stuck to a thin sheet of a material which ischemically aiine to the coating material, or it may be of the samematerial as the coating. The sheet or the coatings can be wetted with asuitable solvent or the corresponding monomer. The nature `of the sheetmaterial used will depend upon the coating, and two examples will begiven of suitable materials. The proportions are parts by weight.

In the first example, the coating material used is that given as Examplel in my aforesaid Application No. 314,563, and is as follows:

Example 1 Copolymer resin of 70% styrene and 30% butyl acrylate 16.40

Chlorinated diphenyl specific gravity 1.530 to 1.630 1.60 Xylene 40.00Butyl acetate 34.00 Ethyl lactate 2.00 Titanium dioxide 3.50 Colloidalsilica 2.50

The single crystal elements with the electrode applied lirmly to thesurface thereof are individually dipcoated with the solution, which onevaporation of the solvents, will provide a film round the element. Therequired thickness of the coating which should be at least 0.0005 inchis achieved from the dipping solution and by adjusting the viscosity andthe temperature of said solution.

The sheet material used in this example is a polystyrene sheet, of about.0015 inch thick. The coated crystal plate is wetted with a styrenemonomer on that side of the crystal which has been electroded, and ispressed against the sheet. A second, and further, crystals are appliedto the same sheet in the same way.

In a second example an acrylic resin is used for coating and cementingthe crystals. The coating composition is that given in Example 3 ofSpecification No. 23965/51, and is as follows:

With this coating composition, the sheet material used can be ofpolymethyl methacrylate, the engaging surfaces being wetted with thecorresponding monomer or with a low boiling point solvent such asmethylene chloride.

The sheet, with the plates secured thereto, can now be handledconveniently as a unit and can be placed on the surface of a work holderand there held by vacuum pressure, suitable holes being formed for thispurpose in the surface.

The plates, whilst being held in this way can be safely machined sincethe adherence to the suport surface is good; the plates are thereforeworked to the desired thickness. During this process it will be seenthat the sides of the plates are protected by the coating against thedeposit of crystal dust upon them.

When the surface has been machined to the desired thickness the surfaceis cleaned and provided with an electrode as above. Thereafter, theplates are separated by cutting the supporting sheet around each plateand plates having suitable orientation are then cemented together bywetting the sections of the supporting sheet adhering to each plate. Thesurplus sheet material is trimmed off, and the pairs of plates arecoated with a coating of the type disclosed in my copending applicationSerial No. 219,756 tiled April 6, 1951.

It will be understood that the customary connecting 4 leads will be madeto the electrodes, and that these leads will be connected together asnecessary. For example, the leads to the main electrodes can be joinedtogether, and those to the outer electrodes joined together to form asuitable transducer element.

The accompanying drawing shows diagrammatically the various steps inpreparing a bimorph as in the method described above. The drawings arenot to scale.

Figure 1 shows in section a crystal plate 10, as it is after roughcutting, by means of a saw, from the crystal blc-ck,

Figure 2 shows the plate .10, in the process of being cut by a miller orgrinder 11, to prepare one surface of the plate,

Figure 3 shows the plate 10 with electrode 12 applied to the workedsurface of the crystal,

Figure 4 shows the plate, with the one electrode thereon, after theinitial moisture-proofing coating 13 has been applied thereto, and

Figure 5 shows two plates, prepared up to the stage of Figure 4, securedto a supporting sheet 14. The sheet is supported upon the surface of awork table 15, which is provided with holes 16 leading to a closed spacebeneath the work table. This communicates through a conduit 17 to asource of vacuum pressure; a control and release valve 18 is included.On the work table the second surface of the members are finished withmiller or grinder 19, and the second electrode 20 is thereafter applied.The plate 10 on the right in Fig. 5 is in the process of being groundand the plate 10 on the left has been ground and the electrode 20applied.

Figure 6 shows the pairs of crystals cemented together and given a finalmoisture-proof coating 21.

l claim:

1. A method of making a piezo-electric assembly which comprises Workinga surface of each of at least two piezoelectric members, applying anelectrode to each of said worked surfaces, applying to each said memberat least over said electrode a coating material, securing each saidcoated member permanently to a supporting sheet of exible materialholding said sheet at against displacement, working the opposite surfaceof each of said members while secured to said supporting sheet,separating the plates by cutting the sheet around the same, andthereafter securing together the sheet material secured to said membersto form said assembly.

2. A method in accordance with claim 1, wherein a further andmoisture-proof coating is applied to said assembly.

References Cited in the tile of this patent UNITED STATES PATENTS2,112,636 Sawyer et al. Mar. 29, 1938 2,386,279 Tibbetts Oct. 9, 19452,440,348 Root Apr. 27, 1948

1. A METHOD OF MAKING A PIEZO-ELECTRIC ASSEMBLY WHICH COMPRISES WORKINGA SURFACE OF EACH OF AT LEAST TWO PIEZOELECTRIC MEMBERS, APPLYING ANELECTRODE TO EACH OF SAID WORKED SURFACES, APPLYING TO EACH SAID MEMBERAT LEAST OVER SAID ELECTRODE A COATING MATERIAL, SECURING EACH SAIDCOATED MEMBER PERMANENTLY TO A SUPPORTING SHEET OF FLEXIBLE MATERIALHOLDING SAID SHEET FLAT AGAINST DISPLACEMENT.